1. Field
The present disclosure relates to the field of olefin polymerization. It more particularly relates to ethylene-propylene copolymers and methods of producing them. Still more particularly, the present disclosure relates to ethylene-propylene random copolymers produced under bulk homogeneous polymerization conditions.
2. Description of the Related Art
Ethylene-propylene (EP) copolymers are typically made in a homogeneous solution type polymerization process. International Patent Publication No. WO 2006/044149, herein incorporated by reference in its entirety, discloses a polymerization process where the polymer dissolution is substantially aided by an inert solvent (“solution process”), such as where the polymerization medium contains more than 60 wt % solvent. The homogeneous solution type polymerization process typically utilizes greater than 60 wt % solvent typically comprising organic compounds of five or more carbon atoms to keep the product polymer in solution and thus to enable fouling-free reactor operations in the production of propylene-containing copolymers. With the accumulated inert propane in the monomer feed, the inert solvent/diluent level is often at or above 80 wt % in the polymerization reactor.
Because solution type processes operate with more than 60 wt % solvent plus the accumulated inerts present in the monomer feed, the monomer and comonomer concentration in the reactor feed is generally 20 wt % or less and is further reduced to less than 2.0 mol/L, or less than 1.0 mol/L, or even less than 0.5 mol/L due to monomer conversion in the polymerization reactor. This low monomer concentration has significant process cost consequences. For example, it reduces catalyst productivity necessitating relatively long residence times in the reactor (typically more than 15 minutes) with a corresponding large reactor volume. The lower monomer concentration also limits the EP copolymer product molecular weight at a given process temperature, which is generally compensated for by applying lower process temperatures (e.g., 35 to 90° C.) at the typical process pressure (typically 1.8 kpsi/124 bar or lower) of the reactor. In general, solution polymerization processes operate at less than 95° C., or less than 90° C., and at less than 2.0 kpsi (13.3 MPa=138 bar), or less than 1.8 kpsi (12.4 MPa=124 bar), or less than 1.5 kpsi (10.3 MPa=103 bar), or even less than 1.0 kpsi (6.9 MPa=69 bar). These low process temperatures in turn make the removal of polymerization heat costly, requiring large refrigeration trains that use a lot of energy and thus are expensive to install and to operate. The high energy usage ultimately also means increased green-house emissions.
As mentioned above, a substantial fraction of the production cost of the prior art solution polymerization process for producing EP copolymers is attributable to the expensive refrigeration systems necessitated by the low polymerization temperature. Another significant associated cost stems from the handling of the inert solvents required to keep the product polymer in solution, to absorb reaction heat, and to keep viscosity low. The handling and recycle of these inert solvents require large investment and substantially add to the operating cost. The solvent residues are also difficult and costly to remove from the product, but must be removed to avoid adversely affecting product quality, and raising environmental concerns related to solvent vapor release to the atmosphere. Consequently, processes enabling higher-temperature operations and/or eliminating the need for inert solvent would be advantageous.
Hence, there is a need for an improved polymerization process for EP copolymers enabling higher-temperature operations and/or eliminating the need for inert solvent/diluent compared to the prior art solution polymerization process for producing EP copolymers. There is also a need for EP copolymer products with improved microstructure and other properties, like, for example lower melt flow rate, compared to EP copolymers produced by the prior art solution polymerization process.